In the field of machine tools (e.g., numerically controlled (NC) lathes), a configuration has been known in which a tool rest capable of carrying a plurality of tools, such as cutting tools, drills, etc., as to be mounted thereon in a parallel arrangement (hereinafter referred to as “a gang tool rest”) is provided, in such a manner that the gang tool rest can be operated for feed-motion along two control axes (e.g., X-axis and Y-axis) orthogonal to each other on a lathe bed. A plurality of tools are mounted on the gang tool rest in parallel to and spaced from each other, with the respective tips or distal ends thereof being oriented in an identical direction. In an NC lathe provided with the gang tool rest, when a workpiece is to be machined with a desired tool on the gang tool rest, the position coordinates of the rotation center axis of the workpiece are applied to the origin of a workpiece coordinate system, and the target position of the tip or distal end of the tool is instructed in the workpiece coordinate system.
During the execution of a series of machining programs for a workpiece, in order to select a tool to be used, in each machining step, from a plurality of tools mounted on the gang tool rest, the gang tool rest is shifted to a retracted location in which the tips of the plural tools do not come into contact with the workpiece, and is operated, as a feed-motion, in a first control-axis direction (e.g., a Y-axis direction) parallel to the laying direction of the tools arranged side-by-side. Then, at an instant when the tip of the selected tool is aligned with the rotation center axis of the workpiece in a second control-axis direction (e.g., an X-axis direction) orthogonal to the first control axis direction, the tool selection process is completed. From this state, the gang tool rest is operated, as a feed-motion, in the second control-axis direction, and the tip or distal end of the selected tool is abutted to the workpiece so as to perform machining. The term “a tip of a tool” or “a distal end of a tool”, as used herein, refers to the part of a tool which first comes into contact with the workpiece in a machining operation.
The plural tools mounted on the gang tool rest may be arranged with the respective tips thereof being regularly positioned to be spaced the same distance from a standard tip position previously set as a characteristic value of the gang tool rest, or may be arranged with the respective tips being irregularly positioned to be spaced by different distances from the standard tip position. In a case where the above-described tool selection method is carried out, in a conventional NC lathe, for a gang tool rest carrying plural tools mounted to have irregular tip positions, the gang tool rest is retracted in the second control-axis direction to a location where the longest tool among all tools mounted on the gang tool rest (i.e., the tool having a largest tip distance between the tip position of each tool and the standard tip position) does not come into contact with the workpiece and, in this state, the subsequent tool selection (or replacement) operation in the first control-axis direction is performed. Alternatively, the gang tool rest may be retracted in the second control-axis direction up to a limit, in the travel of the gang tool rest, permitted in terms of machine construction.
In a case where the tool selection method is carried out, in the conventional NC lathe, for the gang tool rest carrying plural tools having irregular tip positions, the data of the outer diameter of the workpiece, the maximum tip distance from the standard tip position to the tip position of the longest tool, the clearance distance for slightly spacing the tip of the longest tool from the workpiece during tool selection, and the tip position (as coordinates on a second control-axis) of each tool, are provided. After the machining operation using a currently selected tool is completed, an NC device processes this data, so as to move the gang tool rest along the second control axis to the retracted location and in the first control-axis direction to the retracted location and thus selects a next designated tool to be used thereafter, in the manner as described above. In the program of this tool selection method, a plurality of control blocks, such as a coordinate-system shift block for shifting a workpiece coordinate system from a location for the currently selected tool to a location for the next designated tool, a tool-tip positioning block for moving the next designated tool from a selection completing position to a machining position, a coordinate-system restoring block for restoring the workpiece coordinate system to a reference position, and so on, are described subsequently to a tool-number designation block for selecting a tool (due to this designation, tool selection is automatically performed).
The retracting motion of the gang tool rest to a location where the longest tool on the tool rest does not come into contact with the workpiece or a limit location in the permissible travel of the tool rest, conducted to carry out a tool selection for the gang tool rest on which plural tools are mounted at irregular tip positions, is unnecessary step in a feed motion and is essentially useless in a machining operation and results in an increase in the idle time. As a result, if the tool selection is performed many times, the cycle time of the machining program is increased significantly, and a reduction in time of a manufacturing process is thereby obstructed. In the above-described program of the tool selection method in the conventional NC lathe, it is necessary to describe the control blocks, such as the coordinate-system shifting block executed before a machining-step control block, in addition to the tool-number designation block, which has been a considerable burden imposed upon programmers.